The present invention relates to ionic liquids and methods for their preparation. In particular, the present invention relates to ionic liquids that are liquid at relatively low temperatures. Compounds according to the present invention are liquid at temperatures below about 100° C., and are preferably liquid below about 60° C., and more preferably are liquid at or near ambient temperature.
There is currently great interest in the use of ionic liquids as solvents for a wide range of applications. Ionic liquids are low melting point salts that, being composed entirely of ions, posses negligible vapor pressures. By carefully choosing among a wide range of possible cations and anions, ionic liquids may be prepared that are liquid at low temperatures. A number of other solvent properties can be controlled as well, such as polarity and other factors that determine a liquid's suitability as a solvent for a given end-use application.
Conventional organic solvents are high on the list of hazardous chemicals because they are typically volatile liquids that are used in large quantity and produce harmful vapors that are difficult to contain. Ionic liquids, on the other hand, are non-volatile, non-flammable and highly stable solvents, and as such are rapidly emerging as promising replacements for the traditional volatile organic solvents.
Not only do ionic liquids have utility as industrial solvents, they are also suitable, for example, as highly polar solvents for use in preparative chemistry, and as catalysts. The negligible vapor pressure of ionic liquids facilitates product separation by fractional distillation. They also have particular application in electrochemistry, for example, in batteries, fuel cells, photovoltaic devices and in electrodeposition processes.
International Application No. PCT/GB00/01090 discloses ionic liquids that are specific quaternary ammonium salts of zinc, tin and iron halides. The disclosed ionic liquids are reportedly liquid below 60 C and inexpensive to produce. The quaternary ammonium salts of zinc, tin and iron halides are reportedly less water sensitive that earlier prior art ionic liquids, which were quaternary ammonium salts of aluminum trichloride.
Hagiwara et al., J. Fluorine Chem., 99, 1 (1999), and J. Electrochem. Soc., 149, D1 (2002), recently disclosed several ionic liquids comprising various imidazolium fluorides combined with hydrogen fluoride at a specific mole ratio of 1:2.3. Otherwise, prior art salts are minimally electrically conductive, and all are viscous liquids. There remains a need for ionic liquids with greater fluidity for solvent applications and with an electrical conductivity better suited for electrochemical applications.